After a long-announced transition, June 30 marks the end of an era at Microsoft. Well, really it's the end of two eras.

Most people think of Bill Gates, of course, and I'll have plenty more to say about his impending shift to part-time work in the coming days. But another epoch is also really coming to an end--that of Windows XP. As of June 30, large PC makers will no longer be able to sell Windows XP-based PCs, at least on mainstream notebooks and desktops. Retailers will also have only until their current supply is exhausted to sell boxed copies of the operating system.